Mass spectrometer leak detector control circuit



Sept 22, 1970 w. E. BRIGGs ETAL 3,530,291

MASS SPECTROMETER LEAK DETECTOR CONTROL CIRCUIT Filed De. 9, 1965 JI mwo Em v @EN @EN wDOOm mJJwmmm United States Patent O l U.S. Cl. Z50-41.9 3 Claims ABSTRACT OF THE DISCLOSURE Mass spectrometer tube leak detector control with direct reading capability achieved principally by an electron emission regulator which holds electron emission in the spectrometer tube to a desired rate allowing the operator to rely on an initial tuning.

The present invention relates to electrical control circuitry and particularly to a control for a mass spectrometer leak detector.

It is a principal object of the invention to provide a control circuit for a mass spectrometer leak detector, or the like, having a direct reading capability in addition to the usual arbitrary scale.

It is a further object of the invention to provide a zero setting mechanism for such control circuitry having improved stability compared to the conventional dual-potentiometer zero setting mechanisms.

It is a further object of the invention to provide a mass spectrometer control circuit affording a wider range of output current than prior art devices.

It is a further object of the invention to provide a means for indicating operation of the mass spectrometer filament at lower cost than in prior art devices.

The invention therefore comprises the complete electrical circuit combination for achieving all the foregoing objects, and the subcombination therein for achieving one or more of the foregoing objects.

The invention will now be described with respect to a preferred embodiment used for controlling a mass spectrometer leak detector and reference will be made to the accompanying drawing -which is a circuit diagram of the preferred embodiment in such preferred usage.

The mass spectrometer to be controlled is indicated at 100. The instrument comprises a repeller electrode 125, a pair of filaments 111, 112, an ion chamber or grid electrode 120, focus electrodes, 123, 124, an analyzer section indicated as A, a suppressor electrode 220, a collector electrode 201, and the usual intervening grounded shields.

The electrical circuit comprises the principal subcircuits:

(a) A biassing circuit consisting of the resistors 10, 11, 12, 13, 14, -with the adjustable taps 22, 21, 25 and fixed voltage outputs indicated at I and II, and 23;

(b) An emission power supply circuit consisting of the filament transformer 110 with its primary windings connected to a constant voltage regulated A.C. supply and its secondary windings connected to one of the filaments 111, 112;

(c) An emission regulator circuit coupled between the first and second voltage sources I, II and consisting of an emission selector switch 30, rheostats 31, 32, 33, a voltage amplifier and a saturable reactor 101 whose D.C. winding 3,530,291 Patented Sept. 22, 1970 ice 102 is connected as the output of the voltage amplifier and whose A.C. winding 103 is in series with the primary winding of the filament transformer 110;

(d) An output current amplifier comprising tubes 212, 213 and related circuitry, connected to collector 201 and consisting of three stages of amplification with a final cathode follower output having an inverse feedback through resistor 202; and

(e) An ammeter 207 connected to the amplifier via a range switch 205.

Such Conventional circuit elements as power supplies, filters, grounding wires are, in the main, omitted from the drawing for the sake of clarity.

The above-cited biassing means (a) is adjustable through taps 22, 24, 25. However, these taps are only used for tuning and not for sensitvity control. Sensitivity control is achieved through the emission regulator means described below. We have found that a Very effective tuning capability is provided by tying the ion source electrode 120 and suppressor electrode 220 to the same voltage output I. The ion sourcesuppressor bias is varied through tap 22. Adjustment of tap 22 also varies the voltage output II.

It is conventional to have a filament lamp to indicate 'whether the filament is operating. However, it is not feasible to match the impedances of the lamp and filament. Thus, they cannot be placed in series. The prior art answers this problem by using a separate relay for the lamp. The solenoid winding of the relay is in series with the secondary winding of the filament transformer and the filament. According to the present invention, the lamp takes the form of a neon gas discharge bulb 109 and is placed in parallel with the AC. winding 103 of the saturable reactor 101. The bulb is selected to fire at volts, or above.

The emission regulator (switch 30, tubes 40) provides a wide spread of sensitivity. The circuitry used in mass spectrometer leak detectors normally provides four decades of output. By adjusting the emission we attain an additional two decades of output.

The amplifier circuitry (tubes 212, 213, grid resistor 202) used herein is based on a very old design. But the combination of this amplifier (including zero set control and gain control to vary amplification between about 0.1

and 1.0) and the above-described circuitry provides the requisite flexibility for successfully utilizing a direct reading scale-a new result in the leak detector art.

The present invention also provides an improvement in the potentiometer means used for coarse-fine gain control in the amplifier. In the prior art, this is done by dual pots which comprises two potentiometers in series which are adjustable together to provide a variable current with a constant voltage drop between their taps and another potentiometer connected between the taps for fine control. Generally, the coarse control is limited to about a threequarter turn. In the present invention, coarse control is provided by the rheostat 208 and fine control by the p0- tentiometer 209 while the Zener diode 210 provides the requisite voltage regulation. In this preferred embodiment, the diode is selected to break down at 6 volts. The potentiometer means also comprises resistors 233, 234. Resistor 234 is a temperature responsive device whose resistance changes with temperature to stabilize the voltage supply to resistors 209, 211.

The resistor 206 is used to deaden the meter 207 when 3 not in use. However, when the meter is in use, the resistor 206 is shunted out.

In this preferred embodiment of the invention, the circuit elements are selected as follows:

*(Thermistor) temperature coeliicent 0.7 per degree C.

Fixed operating voltages are indicated on the drawing. The drawing also indicates the scale factors corresponding to different settings of the range switch 205. Two scale panels are provided on the instrument-one for arbitrary units and one for direct units. The arbitrary scale panel runs from to 10,000 and the direct scale panel runs from 10X 10d10 to 10 107 standard cc./ sec.

The direct scale panel is used when the switch 30 is set to run current through resistor 31 as shown in the drawing for high emission and high sensitivity. When the switch 30 is moved to resister 32, a medium sensitivity direct scale ranging from 10X109 to 10X106 std. cc./sec. can be inserted in place of the direct reading panel shown in the drawing. When switch 30 is moved to resistor 33, a low sensitivity direct scale ranging from 10 108 to 10X10*5 can be inserted in place of the direct reading scale shown in the drawings. As a practical matter, the maximum meter reading is 6x107 in high, 6 106 in medium and 6x105 in the low sensitivity setting.

OPERATION At the beginning of leak detection operations, the operator evacuates the spectrometer tube 100 to its working pressure range (about 0.1 micron or less). Before tuning the instrument the operator sets the gain control to 100% feedback (by adjusting potentiometer 203) and setting the emission selector switch 30 to the desired operating range (e.g., high emission via connection through resistor 31). The amplifier is zeroed, using the coarse and iine zero switches, as necessary. The filament transformer 110 is then powered by connecting the A.C. supply to its primary winding.

Tuning-The operator then applies a standard helium leak to the spectrometer. He turns the range switch to get the meter needle into the upper portion of the rneter 207. Then he adjusts the taps 22, 24, 2S one at a time to get a peak reading on meter 207. The process is repeated until no additional increase is noted. The tuning can be further advanced by adjusting the ion source magnet of the spectrometer to a peak output reading.

CaZz'bratz'0n.-The reading on the meter is made to agree with the known leak value by adjusting the appropriate emission adjust switch. Where the sensitivity of the instrument is too high, the operator reduces the emission range via selector switch 30. Reduction of sensitivity can also be obtained by increasing the effective pumping speed of the pump acting on the spectrometer tube, such as by removing the throttle usually placed between the spectrometer and pump. Where the sensitivity of the instrument is too low, it is increased by adjusting the gain control (203) to reduce the amplier feedback. 7

`l If the emission adjust switch is used, the operator retunes the instrument to peak output readings and resets the zero as `described above.

Once calibrated, the instrument is ready for direct reading operation. A long series of leak detection cycles can be made without drift from true reading by the direct reading scale panel. This fidelity is in large measure credited to the emission regulator circuit which holds the electron emission to the rate set by switches 30, 31-33. This circuit functions in combination with the bias circuit which has all the bias voltages operating on the peak of the output curve.

If desired, the operator can alternatively use the arbitrary scale panel in the same manner as in prior art leak detection techniques.

What is claimed is:

1. In an electrical circuit for ion generating vacuum devices of the type comprising a thermionic filament, a grid or ion chamber electrode, an ion suppressor and an ion collector electrode, such as mass spectrometer leak detectors and the like, the combination of:

(a) bias means for the device having rst and second voltage outputs, the second voltage output being less than the first output, the first output being connected to the grid and suppressor electrodes, and the second output being connected to the filament,

(b) emission power supply means connected to said lilament,

(c) emission regulator means connected between said iirst and second voltage outputs and including selector means for varying the set Voltage drop between said first and second voltage outputs and a saturable reactor connected between said selector and said emission power supply to vary emission in inverse response to deviations from the selected voltage drop;

(d) an amplifier, with adjustable negative feedback means for adjusting the amplification factor between about 1.0 and about 0.1, the amplilier being connected to the collector electrode, the amplifier comprising a iirst stage amplifier tube with a control grid connected to said electrode and a screen grid connected to potentiometer means; and

(e) a circuit output including a range switch connected to the output of the amplilier.

2. The circuit of claim 1 wherein the zero set potentiometer means of the amplifier comprises a thermistor, a first line setting potentiometer, a second coarse set rheostat, a voltage source, the thermistor being in series between the voltage source and the potentiometer, the potentiometer being in series between the thermistor and the rheostat, and a constant voltage maintaining means connected in parallel across the potentiometer.

3. The electrical circuit of claim 1 further comprising a voltage sensitive element for indicating iilament operation and wherein said element is connected in parallel with the A.C. winding of said reactor.

References Cited UNITED STATES PATENTS 2,663,765 12/1953 Boisblanc 323-74 2,365,611 12/1944 White 315-284 2,604,514 7/1952 Neil 324-33 2,970,303 1/1961 Williams 315-129 3,054,931 9/1962 Inoue 315-284 3,076,139 l/l963 Roberts 315-108 3,346,769 10/ 1967 Sheldon 315-108 WALTER STOLWEIN, Primary Examiner A. L. BIRCH, Assistant Examiner 

